Silicones, polymers with a backbone of silicon-oxygen bonds, are widely used as sorbents, adhesives, lubricants, coatings, and materials for microfluidics, medical devices, and precision molding. Polydimethylsiloxane (PDMS) and other silicones are gas permeable, biologically compatible, optically clear, hydrophobic and biodegradable. They also exhibit good dielectric properties and high thermal and chemical stability, and they swell in organic solvents, especially hydrocarbons. Despite the prevalence of bulk silicone polymers, a scalable synthetic method for producing stable, micron-sized, spherical particles (microspheres) of crosslinked silicones has proven extraordinarily challenging due to the low surface energy of silicone oligomers and their tendency to coalesce and agglomerate during emulsion polymerizations, especially at the elevated temperatures that may be required for polymer curing.
For example, emulsion polymerizations of silicone spheres produce large, polydisperse microspheres ranging in diameter from 50 microns to hundreds of microns with a multimodal size distribution. Other synthetic methods include rapid expansion of a supercritical solution to produce liquid PDMS microspheres, grinding of silicone tubing under liquid nitrogen to form PDMS microparticles, dipping a fiber into noncrosslinked PDMS for “one-at-a-time” PDMS microsphere synthesis, and use of a microfluidic channel to fabricate crosslinked PDMS magnetic microspheres. These methods are cumbersome, have low production rates, and produce only large microspheres (e.g., >100 μm to 1 mm in diameter).
Despite the fabrication challenges, many potential applications for PDMS microspheres have been suggested in the literature. Possible uses include sensors, actuators, and additives for polymer resins. PDMS microspheres may also be suitable as materials for extraction and chromatography and for biomedical applications, such as drug delivery and controlled release. These applications have not yet been well explored, at least in part due to the lack of versatility and control inherent in existing methods to fabricate PDMS microspheres.